SANDSTONES

149

silica bricks. The lower part of what is called the Nattrass Gill
ganister at Gold Hill, Wear head, is not a ganister at all, but a pale
grey sandstone consisting of grains between 0-1 and 0-15 mm.
diameter, cemented by a siliceous cement containing some material
derived from decomposed felspar. In places it is somewhat iron-
stained.
In Durham, the Brigg Hazel rock of Lanehead, Weardale, and
Harperley (p. 44) consists of a fine-grained whitish or greyish
sandstone with grains up to 0-2 mm. diameter, cemented by a small
amount of siliceous matter with some quantity of heavy detrital
minerals. A little chert and felspar occurs at Harperley, whilst
mica occurs in the Lanehead rock. The 'dark grey or blue lower
beds at Harperley are not so valuable, being generally rejected in
favour of the better quality material. The Harperley rock is
rather inferior as a refractory material to that at Lanehead ; it is
entirely used for the manufacture of silica bricks. The Lunedale
rock is a hard, fine-grained sandstone with grains up to 0-2 mm.
diameter, and containing some decomposed felspar and chert, as
well as titanium minerals and ferruginous matter.
The Aid Crag silica rock of Northumberland is a buff-coloured,
soft micaceous sandstone containing about 97 per cent of silica.
The purest rocks are nearly white and are almost free from im-
purities, but most of the best contain mica and some clay, as well
as ferruginous matter. They are generally fairly coarse-grained,
the particles being generally about 0-3-0-4 mm. diameter. This
rock is not much used as a source of sand, but blocks of it are used
for furnace work.
The Carboniferous Limestone ci sand " worked at Waen, near
Mold (p. 44) in North Wales, consists of a soft, white sandstone.
It is readily crushed, and is quite pure, containing, according to
Boswell, about 99 per cent of silica and only about 0-024 per cent
of iron oxide, so that it has practically no iron stains. It is very
uniform in texture, the main portion of the material being of
grains between 0-25 and 0-5 mm. diameter. There is also about
12 per cent of grains less than 0-25 mm. diameter and up to 10 per
cent of grains over 0-5 mm. diameter. Much of the finest material
can readily be removed by washing. There is very little heavy
detrital material present in the material, that which does occur
consisting chiefly of ilmenite partly converted into leucoxene,
anatase, tourmaline, zircon, and rutile. Some decomposed felspar
also occurs. This " sand " is principally used for the manufac-
ture of soap, and has been suggested as suitable for glass-making
and various refractory purposes, such as linings of open-hearth
furnaces.
The sand at Miner a (p. 47), which belongs to the same forma-
tion, is rather coarser and less uniform in texture. In a sample
examined by Boswell about 17-18 per cent of grains were over
1 mm. diameter, and a total of nearly 50 per cent exceeded 0-5 mm.,
as distinct from less than 10 per cent in the case of the Mold